1. Field of the Invention
The present invention relates to a magnetic garnet single-crystal film (Bi(bismuth)-substituted rare earth-iron garnet single-crystal film) and a method of producing the film, and to a Faraday rotator comprising the film.
2. Description of the Related Art
Bi-substituted magnetic garnet single-crystal films for Faraday rotators such as optical isolators, optical circulators and others of ten crack while they grow in a mode of liquid-phase epitaxial growth on a substrate or while they are polished and worked into such Faraday rotators, and are therefore problematic in that the yield in working them into Faraday rotators is extremely low. To solve the problem, for example, a method is disclosed in Japanese Patent Laid-Open No. 139093/1992(hereinafter referred to as Reference 1), in which a liquid-phase epitaxial film is grown on a substrate in such a controlled manner that the lattice constant of the growing film may be the same as that of the substrate at room temperature to thereby prevent the film from cracking. Another method is disclosed in Japanese Patent Laid-Open No. 92796/1994 (hereinafter referred to as Reference 2), in which the liquid-phase epitaxial growth of film is so controlled that the lattice constant of the growing film is gradually increased in the direction of the film growth from the film-substrate interface to thereby prevent the film from cracking.
Bi-substituted magnetic garnet single-crystal films formed in a liquid-phase epitaxial growth process often crack while they grow at a temperature between 700 and 1000° C., or while the thus-grown films are cooled, or while they are polished to be worked into Faraday rotators. The reason is because of the difference in the thermal expansion coefficient between the films and the gadolinium-gallium-garnet (Gd3Ga5O12)-type single-crystal substrates (hereinafter referred to as CaMgZr-substituted GGG single-crystal substrates) for them.
After having grown on the substrate, the Bi-substituted magnetic garnet single-crystal film is polished and worked at room temperature into Faraday rotators. In order to prevent the single-crystal film from cracking while it is worked so, the film must be so controlled that its lattice constant around the interface between the film and the underlying substrate may be nearly the same as that of the substrate. However, the thermal expansion coefficient of the Bi-substituted magnetic garnet film is larger by around 20 to 30% than that of the substrate. Therefore, when the single-crystal film is so controlled that its lattice constant around the interface between the film and the substrate may be the same as that of the substrate at room temperature, then the lattice constant of the film shall be larger than that of the substrate at a temperature of from 700 to 1000° C. at which the film grows on the substrate. As a result, while growing at such a temperature, the substrate and the single-crystal film on the substrate will warp to have a convexedly curved profile to the film side.
In case where the single-crystal film is grown on a substrate in such a controlled manner that the lattice constant of the film may be the same as that of the substrate at room temperature and that the lattice constant of the overall structure may have a predetermined value, as in Reference 1, the growing film warps more to have a more convexedly curved profile while growing more to have an increased thickness, and warps most when the thickness of the growing film has reached nearly a half of the thickness of the substrate. After having grown further more so that its thickness is over nearly a half of the thickness of the substrate, the film does not warp any more, but its surface cracks to have concentric circular cracks. As a result, the yield in working the film into Faraday rotators is low.
In case where the single-crystal film is grown on a crystal substrate of which the thickness is at least about 2 times the necessary thickness (thickness of Faraday rotator+depth of film to be ground away) of the film to be worked into Faraday rotators, it is prevented from being damaged to have concentric circular cracks. However, if the crystal substrate is thick, the single-crystal film growing or having grown on it often cracks in the substrate-film interface while the film grows or while the grown film is cooled, depending on the condition under which the film is grown and on the varying lattice constant of the substrate that may cause a minor difference between the lattice constant of the substrate and that of the film in the substrate-film interface. This causes the reduction in the yield in working the film into Faraday rotators.
Therefore, as proposed in Reference 1, while the growth of single-crystal film is attempted on a substrate in such a controlled manner that the lattice constant of the single-crystal film may be the same as that of the crystal substrate at room temperature and further that the lattice constant of the overall structure may have a predetermined value, the concentric circular cracks are formed on the surface in the condition that the crystal substrate is relatively short in thickness, and other cracks are formed on the substrate-film interface in the condition that the crystal substrate is relatively long in thickness. As a result, it is difficult to avoid the problem of low yield in working the film into Faraday rotators.
In the method described in Reference 2, the lattice constant of the single-crystal film growing on a substrate is gradually increased with the increase in the thickness of the growing film to thereby prevent the film surface from cracking to have concentric circular cracks. If, in the substrate-film interface, the lattice constant of the single-crystal film is controlled to be the same as that of crystal substrate at room temperature, the substrate with the film growing thereon will warp to have a convexedly curved profile at the temperature at which the film is growing thereon. Therefore, the lattice constant of the single crystal of the film is increased with the increase in the thickness of the growing single-crystal film so that the growing film may have a convexedly curved profile in accordance with the warped profile of the substrate.
In that manner, the problem of concentric circular cracks of the single-crystal film growing on a thin substrate in the method described in Reference 1 can be solved. In addition, when the single-crystal film is grown on a thin substrate in the method of Reference 2, it is free from the problem of cracking in the substrate-film interface that may occur when the film is grown on a thick substrate. Accordingly, as compared with the method of Reference 1, the method of Reference 2 is effective for preventing film cracking in the step of growing and cooling the single-crystal film.
However, if the single-crystal film is prevented from cracking according to the operation as above, the film formed shall have a convex profile, and will be still kept convexedly warped even after cooled to room temperature. Regarding its profile, the crystal substrate is in the form of a flat disc. Therefore, if a convexedly-warped, magnetic garnet single-crystal film is epitaxially grown on the crystal substrate, the interface between the substrate and the film involves intrinsic stress, and the film will be convexedly warped in some degree at room temperature. Accordingly, while polished and worked, the single-crystal film will be cracked owing to the intrinsic stress. As a result, the yield in working the film into Faraday rotators is low.